The typical current procedure for cassette-loading microcomponents is to deliver the components on carrier and transport devices so that they can be picked up from the carrier for the assembly process by means of special gripping tools. The individual components are delivered at a defined spacing depending on their structural shape, e.g. on tapes that are adhesive on one side (blue tape), or they are fixed in the proper position by gel in gel packs (F+M Feinwerktechnik Mikrotechnik Mikroelektronic 105, 1997, 43-45). Other methods consist of placing the microcomponents in the correct position for gripping in chessboard-like indentations of rectangular tablets or in modularly constructed cassettes. (41st International Scientific Council of the Technical University [TU] of IImenau, Sep. 25, 1996). The ordering system allows for defined gripping or removal of these parts from the carrier.
The drawback with these methods, however, is that frequently the ordering system of the production process is not used for the relative positioning and ordering of the parts. Instead the microcomponents are initially delivered in bulk and are then placed onto the aforementioned conveyors in the correct position ready for gripping, which is a time-consuming process. As a result, an intermediate step after the production process is required for cassette loading, the complexity of which is significant and comparable to the subsequent microassembly step.
To simplify the handling of microcomponents, so called foil cassettes were developed, which are described in the German Laid Open Publication DE-OS 197 09 136. These are disk-shaped plates in which the microcomponents are integrated in such a way that they are enclosed along their lateral surfaces in a form-fitting manner. The disk-shaped plate, or the cassette material surrounding the microcomponents, thus forms the component carrier. These foil cassettes with microcomponents are produced by first forming elevations on a component base plate made of the same material by means of vacuum casting, injection molding, reaction molding, or hot stamping. Subsequently, the microcomponents are encapsulated by means of a molding material that solidifies. Thereafter, the microcomponent base plate and, where applicable, the molding material covering the microcomponents, is removed so that the end faces of the microcomponents are exposed.
To assemble the microcomponents, the cassette is grasped and positioned such that the respective microcomponent to be mounted is at the intended location where it is e.g. connected with another microcomponent. To this end, the microcomponent must be pushed out of the foil cassette and pressed onto the existing microcomponent.
For foil-cassette loaded components, various preparation steps are required prior to the actual assembly process as a function of the component geometry and the assembly task.
In the case of ring-shaped or sleeve-shaped components, cassette cores must be removed. It is not possible to eject these cores during assembly by means of the structures of the mating component particularly if the mating component is made of plastic because its buckling strength is not sufficient. Due to the relatively large forces that must be applied to separate the component from the cassette, the cores must be removed by means of metal tools prior to the assembly process.
The ejection of microcomponents from foil cassettes can in part require considerable forces. These forces are highest at the beginning of the ejection process, since the microcomponents must first be detached from the cassette material. Microroughnesses, which must be sheared off by the relative movement between component and cassette, are presumed to be the cause. At the same time, a stick/slip effect is observed, i.e. after static friction has been overcome during ejection, the lower sliding friction occurs.
To reduce the ejection force during the assembly process, the microcomponents, which are removed from the cassette in assembly direction, are therefore first partially pushed out of the cassette in preparation of assembly.
To eject the microcomponents from the cassette, component-specific tools (ejector pins) are required.
The maximum cross-sectional area of the tool is determined by the size of the end face area of the microcomponent and the tolerance of the assembly machine. For instance, in the case of circular cross sections, the maximum tool diameter corresponds to the smallest diameter of the end face minus twice the positioning accuracy of the machine. The minimum length of the ejection tool is determined by the thickness of the foil cassette. With increasing height of a microcomponent, the adhesive forces to be overcome are consequently greater and the ejector pin must be longer, so that the buckling resistance of the tool is reduced. This sets a physical limit to the usability of foil cassette loading with respect to the realizable aspect ratio.
The component geometry dictates the removal direction for the ejection of the microcomponent from the cassette, whereas the assembly task determines the assembly direction.
Components that can be removed in assembly direction can be assembled directly from the cassette with the mating components. Components that can be assembled directly are cylindrical components and those whose thickness increases in assembly direction.
In contrast, components that taper in assembly direction must first be removed from the foil cassette opposite to the assembly direction. To this end, the microcomponents must be fixed in a component-specific device, e.g. by applying a vacuum. This device serves for interim storage until the assembly mates receive the components or, in the case of basic components, until the subassembly is finished. The handling of such components is therefore substantially more complex than for components that can be removed in assembly direction.